Download IPHAS: Surveying the Northern Galactic Plane in Hα

THE ING NEWSLETTER
No. 9, March 2005
SCIENCE
IPHAS: Surveying the Northern Galactic Plane in Hαα
J. E. Drew1, D. J. Lennon2, R. Greimel2, A. Zijlstra3, J. Irwin4, A. Aungwerowijt 5, M. J. Barlow5,
R. Corradi2, C. J. Evans 2, B. Gaensicke6, P. Groot7, A. Hales 5, E. Hopewell1, M. J. Irwin4,
M. Jaigirdar 2, C. Knigge8, P. Leisy 2, A. Mampaso 9, M. Matsuura3, L. Morales Rueda7, R. Morris 10,
Q. A. Parker11, S. Phillipps10, P. Rodríguez Gil9, G. Roelofs7, I. Skillen2, D. Steeghs12, Y. C. Unruh1,
K. Viironen 9, J. Vink1, N. A. Walton4, A. Witham 8, N. Wright5, A. Zurita13
1: Imperial College London; 2: Isaac Newton Group of Telescopes; 3: Manchester University; 4: CASU, Cambridge; 5: University
College London; 6: Warwick University; 7: Nijmegen University, The Netherlands; 8: Southampton University; 9: Instituto de
Astrofísica de Canarias; 10: Bristol University; 11: Macquarie University, Australia; 12: Harvard-Smithsonian Centre for Astrophysics,
USA; 13: Granada University, Spain.
H
α emission is ubiquitous in
our Galaxy. It traces ionised
gas of assorted nebulae such
as HII regions, planetary nebulae,
Wolf-Rayet nebulae, and supernova
remnants. It is a strong signature of
active stars, interacting binaries, very
massive stars (especially supergiants,
Luminous Blue Variables and WolfRayet stars), Be stars, post-AGB stars,
pre-main-sequence stars and so on.
These objects represent important
evolutionary phases which are
generally short lived, and are therefore
few in number and difficult to find.
Their discovery is therefore well
worth the effort of a concerted
programme and in August 2003 a
major new survey project was started
using the Wide Field Camera (WFC)
on the Isaac Newton Telescope (INT)
to do just that. It is called the INT
Photometric Hα Survey of the Northern
Galactic Plane, or IPHAS for short.
IPHAS is a collaborative UK/NL/ES
venture led by Janet Drew in UK, Paul
Groot in The Netherlands, and Antonio
Mampaso in Spain. Its goal is to
conduct an Hα survey of the entire
northern Galactic Plane in the latitude
range – 5°< b< +5°, a sky area of 1800
sq. deg., covering the magnitude range
13 < r'< 20. That such a survey is
needed can be deduced from Figure 2,
and when complete it will represent an
enormous improvement over previous
work. For example it is expected that
in excess of 10,000 new emission line
Figure 1. The crescent nebula, NGC6888, which surrounds the Wolf-Rayet star PPM
84423, seen in Hα emission by IPHAS. Colour image courtesy of Jonathan Irwin.
stars alone will be discovered, an order
of magnitude increase on known
sources.
Survey Details
The 1800 square degrees will be tiled
with 7635 pointings of the WFC, each
of which is paired with a second
pointing at an offset of 5 arcmin W and
S. The purpose of the offset field is to
ensure that we cover stars in the gaps
between detectors, but also means that
the majority of sources are observed at
least twice. Each field is to be observed
with the Sloan r' and i' filters, plus a
narrow band Hα filter, with exposure
times of 30s, 10s and 120s respectively.
The use of a guide star is not necessary
for such short exposure times and is
dispensed with to save on overheads,
while for each Hα – r'– i' sequence at
a given pointing the CCD readout,
filter movements, and final telescope
movement, are overlapped for
additional savings. Finally, human
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No. 9, March 2005
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Figure 2. The left-hand panel illustrates the effect of a simple extrapolation of numbers of catalogued emission line stars from
previous Hα surveys of Kohoutek & Wehmeyer (1999) in the north, and Stephenson & Sanduleak (1971) in the south. Based on
a rough extrapolation of these earlier surveys it is estimated that of order ~10 4 emission line stars will be discovered. The central
and right-hand panels compare the quality of the current IPHAS colour-colour diagrams with those for the same sky area taken
from the UK Schmidt southern Hα photographic survey. Notice the superb delineation of the field population of normal stars and
M dwarfs in the IPHAS data (cf. Figure 3).
interaction is avoided as much as
possible by the use of automatic
observing scripts, which typically run
for about 3 hours, broken only to take
standard star observations.
All data are transferred using external
discs to the Cambridge Astronomical
Survey Unit (CASU) where they are
processed using the CASU pipeline;
calibration steps consisting of bias
correction, flat-fielding, de-fringing,
astrometric solution, flux calibration
and object catalogue generation (see
the article by Irwin et al. on WFS in
this issue). Work is progressing on the
construction of a unified object
catalogue and flux calibration for the
complete survey, with an expected
first data release of final data products
expected in early 2006. It is hoped that
completion of the full survey will occur
during 2006, with the full data release
coming during 2007.
Some Expected Science
Strong Hα emission line stars are of
course reasonably easily selected from
the sample since they are well
separated from the bulk population in
the (r'– i', r'– Hα) diagram. Indeed an
initial blue selection of such objects has
already been made using the 2003 data
and some subsequent follow-up
spectroscopy has revealed the vast
majority of these to be Be-like stars,
with a sprinkling of CVs, compact
PN, potential luminous blue
supergiants and the odd QSO (Figure 3).
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Figure 3. Sampling the Hα zoo as discovered by IPHAS. Clockwise from top left; a
typical Be star, a cataclysmic variable, a potential luminous blue supergiant in Cygnus
and a low redshift QSO (the QSO was detected as a peculiar outlier in the colourcolour plane).
Not all emission line objects are as
easily discovered however and it is
crucial that effort is put into
understanding the IPHAS colour-colour
plane so that future object selections
can be made with some confidence.
This problem has been attacked on
two fronts; through the construction of
synthetic photometry using a library of
observed flux-calibrated stellar spectra,
and by conducting spectroscopic
surveys of selected fields as a visual
check on predicted spectral types.
Figure 4 illustrates the result of this
process for a sample field in Cygnus.
Several thousand spectra in a number
of selected fields have now been
amassed using Hectospec on the MMT
and AF2 on the WHT. In general this
demonstrates that the synthetic
photometry is well matched to the
observations, although detailed studies
of this field, and additional test fields
indicate that some further investigation
of the calibration for very red objects
is required.
IPHAS is also producing new
discoveries of nebulae, one of the first
to be discovered has been named after
the wedding of Su Alteza Real El
Príncipe de Asturias Don Felipe de
Borbón with Doña Letizia Ortiz
Rocasolano which took place around
THE ING NEWSLETTER
No. 9, March 2005
Figure 4. Left-hand panel shows the simulated positions of the unreddened main sequence (black), giant (red) and supergiant
(red) stars in the (r'– i',r'–Hα) plane according to spectral type. The right-hand panel shows the observed sources in a field in the
Cygnus region (black points), with some spectroscropically determined spectral types overplotted in colour; light blue points are
early-type stars, red points are mostly G and K stars, while dark blue points are M stars. The magenta points are strong Hα
absorbers, in fact they are white dwarfs, while the green points are emission line stars, in fact Be stars. Note that the trend from
early-type to late-type stars is well reproduced by the synthetic colours (there is a zero point offset due to extinction which drives
stars diagonally upward and to right this diagram). The central gap in the spectroscopic sample is simply a selection effect due to the
algorithm used to select targets for follow-up. All spectroscopy for this field was carried out using Hectospec on the MMT.
the time of the PN’s discovery. PNG
126.62+1.32 is a rare quadrupolar
nebula, and it was spectroscopically
confirmed as a PN in 2004 using the
WHT. A deeper image, compared to the
discovery image, is shown in Figure 5,
which illustrates the initially
undetected faint lobes of the nebula.
Besides looking for new nebulae,
IPHAS has proved to be exceptionally
useful for taking a new look at
previously known nebulae; ING users
will be familiar with the image of the
Wolf-Rayet NGC6888 which adorned
the ING Christmas card in 2004
(Figure 1). However Figure 6 shows
the spectacular 5° × 3.5° mosaic of
images covering the supernova
remnant S147. Bear in mind that this
image, which is binned for the purposes
of this article, has immense detail on
the arcsecond scale.
To date the survey is approaching its
half-way point in terms of completed
fields. However as we look forward to
the completion of the northern
hemisphere survey it’s clear that we
are already seeing new discoveries, and
generating additional exciting ideas
for follow-up science and data mining.
For example the colour-colour plane
morphologies across the Galactic Plane
will provide useful insights into the
structure of the northern Milky Way,
while linking IPHAS photometry with
JHK survey data from 2MASS and the
UKIDSS Galactic Plane Survey will add
further powerful diagnostic capabilities.
Survey area: ~1800 sq deg in 2×7600 overlapping
pointings
Region: – 5° < b < +5° , 25° < l < 225°
Depth: 13 < r' < 20
We thank the many observers who
have contributed to this programme.
Finally, for more complete details
interested parties should refer to the
first survey paper which is currently
in preparation (Drew et al., 2005, to
appear in MNRAS), and to the IPHAS
home page at http://astro.ic.ac.uk/
Research/Halpha/North/. ¤
Seeing: < 1.7 arcsec
WFC pixel size: 0.33" × 0.33"
WFC field size: ~0.25 sq deg in four 2k×4k CCDs
Filters: r' : λc=6420 Å, FWHM = 1347 Å, t exp =30 s
i' : λc= 7743Å, FWHM = 1519Å, t exp=10s
Hα: λc=6568 Å, FWHM= 95 Å, t exp =120 s
IPHAS Home:
http://astro.ic.ac.uk/Research/Halpha/
North/index.html
Janet Drew ([email protected])
Table 1. IPHAS essentials.
Figure 5. Left: The newly discovered Planetary Nebula
PN 126.62+1.32, the ‘Prince of Asturias’ is a rare
quadrupolar nebula (central region), with extended
fainter lobes extending over 16 arcminutes from the
central star. Top: Publicity version of the discovery image.
Figure 6 (next two pages). A 5° × 3.5° mosaic of the
supernova remnant S147 in Hα. North is to the top
and East to the left. Courtesy of Albert Ziljstra and
Jonathan Irwin.
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